Brushless DC motors are particularly suited for applications requiring high start-up torque and speed control. Exemplary of such an application is in a disk drive where a brushless DC motor is used to quickly bring a disk up to a rotational speed at which data can be transferred to or from the disk and, once at the defined rotational speed, to maintain the rotational speed for the duration of any data transfer.
Among the elements of a brushless DC motor is a bridge which is used to generate a commutated magnetic field. The bridge is typically comprised of a plurality of coils or windings that are electronically connected to one another such that current can flow in either direction through each coil or winding to produce the commutated magnetic field. The commutated magnetic field provides a unipolar torque for rotating a rotor. The commutated magnetic field is produced by first selecting one coil of the bridge to receive current from a DC power supply for a defined amount of time while "deselecting" or not providing current to the other coils of the bridge. In response to the current, the selected coil creates a stationary magnetic field. The stationary magnetic field is then, in effect, commutated by selecting another coil of the bridge to receive current from the supply for a defined amount of time while "deselecting" the previously selected coil and all other coils. The process of selecting and "deselecting" coils is known as commutation and the state of all of the coils, selected or deselected, during the defined amount of time when one of the coils is selected is known as a commutation state.
Included in a brushless DC motor, for control, is bridge drive circuitry that is used to select one coil and deselect all the other coils. Typically, switches located at the terminals of the coils comprise the driver circuitry. To select a particular coil, a switch located at each terminal of the coil to be selected is placed in an "ON" condition. In contrast, a coil is "de-selected" by placing at least one of the aforementioned switches associated with the coil in an "OFF" condition. In addition to being used to select a particular coil to receive current from the DC power supply, the switches are also used to regulate the current supplied to a selected coil which, in turn, controls the torque and, hence, the rotational speed of the motor. Typically, regulation of the current supplied to a selected coil is achieved by a switch mode regulation technique, like pulse width modulation or pulse amplitude modulation.
The brushless DC motor also includes a rotor for, among other things, providing a permanent magnetic field. Interaction of the permanent magnetic field produced by the rotor and the commutated magnetic field produced by the bridge causes rotation of the rotor.
The brushless DC motor also includes sensors for providing signals indicative of the location of the permanent magnetic field relative to the bridge. These signals allow a commutator to control the driver and, hence, the bridge so that the torque necessary to achieve rotation of the rotor is produced. The signals produced by the sensor are also indicative of the rotational speed of the rotor and, as such, are used to measure the rotational speed of the rotor.
Also included in the brushless DC motor, for control, is a commutator which generates, in response to the signals produced by the sensors, a signal or signals that are applied to the drive circuitry to commutate the coils of the bridge in a manner that interacts with the permanent magnetic field and thereby causes the rotor to turn. The commutator determines which switch or switches associated with a selected coil are to be used to regulate the current applied to the selected coil. As previously mentioned, regulation of the current provided to the coils of the bridge determines the torque delivered by the motor. Control of the torque, in turn, determines the speed of the motor for a given load.
Presently, the manner in which commutators control the driver circuitry and, hence, the bridge results in the DC power supply being forced to sink the current produced by a coil after it changes from a condition where it is being provided with current to a condition where it is not being provided with current. More specifically, when a coil is provided with current it establishes a magnetic field which, in essence, is an energy reservoir. When the coil is no longer being provided with current the magnetic field decays and, in so doing, generates a current in the coil. Forcing the DC power supply to sink current is commonly know as backdriving the power supply. Most, if not all, of the commercially available DC power supplies used with brushless DC motors are not designed to sink current. Consequently, backdriving of such a supply places the supply in non-specified mode of operation which, in turn, affects the operation of the motor. Moreover, backdriving of the supply generally forces the supply to shut itself and, consequently, the motor down. In addition, the backdrive signal produced by a coil produces electromagnetic interference (EMI) that adversely affects the performance of other circuitry in, for instance, a disk drive apparatus.
One of the situations where the control provided by presently known commutators results in backdriving of the DC power supply is where both switches associated with a selected coil are pulse width modulated, i.e. alternately "chopped ON" and "chopped OFF", to regulate the current provided to the selected coil by the power supply. Backdriving occurs in this situation, if both switches are "chopped OFF" at the same time during current regulation of the selected coil. Backdriving also occurs if pulse amplitude modulation, rather than pulse width modulation, is used to regulate the current.
To eliminate the backdriving that occurs during a commutation state when both switches associated with a selected coil are "chopped OFF" at the same time, commutators have been developed which use only one of the switches associated with a selected coil to regulate the current. Unfortunately, if the switch used to regulate current to a selected coil is also used to regulate the current to the coil that is selected in the subsequent commutation state, and if the switch is "chopped OFF" at the instant of commutation between the selected coils, then backdriving of the power supply occurs.
In an effort to eliminate all backdriving of the DC power supply due to energy stored in the coils of the bridge, series connected resistor-capacitors circuits have been connected in parallel with each of coils of the bridge. These series RC circuits are typically known as snubbers. While snubbers do absorb some of the current which would otherwise backdrive the DC power supply, a prohibitively large capacitor is required to prevent all backdriving of the supply due to energy stored in a coil. Unfortunately, snubbers also shunt some of the current which would otherwise be supplied to the selected coil during a commutation state. Consequently, there is power dissipation in the snubber which ages the snubber and reduces the efficiency of the motor, including the bridge driver circuitry.
Based on the foregoing, there is a need for an apparatus and method of commutating brushless DC motors that improves upon the known state of the art with respect to preventing backdriving of the DC power supply.